U.S. patent number 10,551,466 [Application Number 15/861,888] was granted by the patent office on 2020-02-04 for correction of a magnetic resonance transmission signal.
This patent grant is currently assigned to Siemens Healthcare GmbH. The grantee listed for this patent is Jurgen Nistler. Invention is credited to Jurgen Nistler.
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United States Patent |
10,551,466 |
Nistler |
February 4, 2020 |
Correction of a magnetic resonance transmission signal
Abstract
The disclosure relates to a method for determining a transfer
function of a transmitting system of a magnetic resonance device,
to a method for the correction of a transmission signal of a
magnetic resonance device, to a corresponding magnetic resonance
device, and to a computer program product for carrying out the
method. The method includes determining a transfer function using a
transmission characteristic of a transmitting system of the
magnetic resonance device, wherein the transfer function is
frequency-dependent. A transmission signal may be corrected using
the transfer function. An excitation pulse may be emitted by the
transmitting system using the corrected transmission signal.
Inventors: |
Nistler; Jurgen (Erlangen,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Nistler; Jurgen |
Erlangen |
N/A |
DE |
|
|
Assignee: |
Siemens Healthcare GmbH
(Erlangen, DE)
|
Family
ID: |
62636613 |
Appl.
No.: |
15/861,888 |
Filed: |
January 4, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20180196114 A1 |
Jul 12, 2018 |
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Foreign Application Priority Data
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Jan 12, 2017 [DE] |
|
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10 2017 200 446 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R
33/583 (20130101); G01R 33/5608 (20130101); G01R
33/5659 (20130101); G01R 33/4835 (20130101); G01R
33/445 (20130101) |
Current International
Class: |
G01R
33/56 (20060101); G01R 33/565 (20060101) |
Field of
Search: |
;324/309 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102012210280 |
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Dec 2013 |
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DE |
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102013226170 |
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Jun 2015 |
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DE |
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Other References
Kuznetsov, Yury, et al. "The ultra wideband transfer function
representation of complex three-dimensional electromagnetic
structures." 34th European Microwave Conference, 2004.. vol. 1.
IEEE, 2004. (Year: 2004). cited by examiner .
Gaikovich, K. P., et al. "Rectification of near-field images."
Proceedings of 2002 4th International Conference on Transparent
Optical Networks (IEEE Cat. No. 02EX551). vol. 1. IEEE, 2002.
(Year: 2002). cited by examiner .
Lebsack, Eliot T., and Steven M. Wright. "Iterative RF pulse
refinement for magnetic resonance imaging." IEEE transactions on
biomedical engineering 49.1 (2002): 41-48. (Year: 2002). cited by
examiner .
German Office Action for related German Application No. 10 2017 200
446.0 dated Sep. 22, 2017. cited by applicant.
|
Primary Examiner: McAndrew; Christopher P
Attorney, Agent or Firm: Lempia Summerfield Katz LLC
Claims
The invention claimed is:
1. A method for determining a transfer function of a transmitting
system of a magnetic resonance device, the method comprising:
determining the transfer function using a transmission
characteristic of the transmitting system of the magnetic resonance
device, wherein the transfer function is frequency-dependent, and
wherein the transfer function is configured to correct a
transmission signal having a transmit frequency that does not match
a center frequency of the transmitting system.
2. The method of claim 1, wherein the transfer function is
determined using an absorbed power.
3. The method of claim 1, wherein the transfer function is
determined using an input reflection factor R(f), wherein the
transfer function is proportional to a function:
(1-|R(f)|.sup.2).sup.1/2.
4. The method of claim 1, wherein the transfer function is
determined using a current I(f) measured in a radio frequency (RF)
transmitting antenna of the magnetic resonance device, wherein the
transfer function is proportional to a function: I(f)/U.sub.tra(f),
where U.sub.tra(f) is a RF transmitting voltage.
5. The method of claim 1, wherein the transfer function is
determined using at least one pickup probe, wherein the transfer
function is proportional to a function: U.sub.pu(f)/U.sub.tra(f),
where U.sub.pu(f) is a voltage induced in the at least one pickup
probe and U.sub.tra(f) is a RF transmitting voltage.
6. The method of claim 1, wherein the transfer function is
determined using a forward voltage and a reflected voltage in a
power supply unit to a RF transmitting antenna, wherein the
transfer function is proportional to a function:
U.sub.pu,coupling(f)/U.sub.tra(f), where U.sub.pu,coupling(f) is
obtained from a measurement of a complex addition of the forward
voltage and the reflected voltage, and U.sub.tra(f) is a RF
transmitting voltage.
7. The method of claim 1, wherein the transfer function is
determined by a variation in an applied magnetic field, wherein the
transfer function is proportional to a function: 1/U.sub.tra(f),
where U.sub.tra(f) is a RF transmitting voltage adjusted for the
applied magnetic field.
8. The method of claim 7, wherein the RF transmitting voltage is
adjusted in a case of different magnetic field strengths such that
an amplitude of a magnetic resonance signal remains constant.
9. The method of claim 8, wherein the magnetic resonance signal
originates from just one plane.
10. The method of claim 1, wherein the transfer function is
determined using a simulation.
11. A method for correction of a transmission signal of a magnetic
resonance device, the method comprising: providing a transfer
function that has been determined using a transmission
characteristic of a transmitting system of the magnetic resonance
device, wherein the transfer function is frequency-dependent; and
correcting the transmission signal using the transfer function, the
transmission signal has a transmit frequency that does not match a
center frequency of the transmitting system.
12. The method of claim 11, further comprising: emitting an
excitation pulse by the transmitting system using the corrected
transmission signal such that a uniform flip angle excitation
occurs due to the emitted excitation pulse.
13. The method of claim 12, wherein the transfer function is
standardized, and the excitation pulse is corrected using the
standardized transfer function.
14. The method of claim 12, wherein the transfer function is
transformed, and the excitation pulse is corrected using the
transformed transfer function.
15. The method of claim 11, wherein an applied magnetic field is
less than 1 T.
16. The method of claim 11, wherein the transmitting system
comprises at least one superconducting RF transmitting antenna.
17. The method of claim 11, further comprising: exciting a
plurality of slices simultaneously with the transmitting
system.
18. The method of claim 11, wherein the transmitting system excites
a field of view, wherein the field of view has an extent in a first
direction, wherein a gradient magnetic field is applied in the
first direction, and wherein a product of the extent of the field
of view of the first direction and an amplitude of the magnetic
gradient field in the first direction is greater than 2 mT.
19. A magnetic resonance device comprises: a correction unit having
one or more processors configured to determine a transfer function
using a transmission characteristic of a transmitting system of the
magnetic resonance device, wherein the transfer function is
frequency-dependent; and a storage unit configured to store the
transfer function, wherein the transfer function is configured to
correct a transmission signal having a transmit frequency that does
not match a center frequency of the transmitting system.
20. A computer program product having a program configured to be
loaded directly into a storage device of a programmable arithmetic
unit of a correction unit of a magnetic resonance device, the
computer program product configured to, when the program is run in
the programmable arithmetic unit of the correction unit, at cause
the magnetic resonance device to at least perform: determine a
transfer function using a transmission characteristic of a
transmitting system of the magnetic resonance device, wherein the
transfer function is frequency-dependent, wherein the transfer
function is configured to correct a transmission signal having a
transmit frequency that does not match a center frequency of the
transmitting system.
Description
The application claims the benefit of German Patent Application No.
DE 10 2017 200 446.0, filed Jan. 12, 2017, which is hereby
incorporated by reference in its entirety.
TECHNICAL FIELD
The disclosure relates to a method for determining a transfer
function of a transmitting system of a magnetic resonance device,
to a method for the correction of a transmission signal of a
magnetic resonance device, to a corresponding magnetic resonance
device, and to a computer program product for carrying out the
method.
BACKGROUND
Magnetic Resonance Imaging (MRI) is a known examination technique
for generating images of the inside of a body of a patient, and is
based on the physical phenomenon of magnetic resonance (MR). A
magnetic resonance device includes, for this purpose, a
transmitting system, with which radio frequency (RF)
electromagnetic excitation pulses, (also called RF pulses), may be
generated which are irradiated into the patient during a magnetic
resonance scan. U.S. Patent Application Publication No.
2014/0347054 A1, U.S. Pat. No. 8,901,929 B2, and German Patent No.
DE 10 2012 210 280 B4 disclose, by way of example, various
embodiments of a transmitting system. From the RF irradiation, a
magnetic alternating field results having a transmit frequency,
which is also called the B.sub.1 field. The irradiated excitation
pulses are capable of deflecting nuclear spins in order to obtain a
desired flip angle distribution for the respective examination. The
deflected nuclear spins in turn emit MR signals, which are measured
by the magnetic resonance device.
The transmitting system includes at least one RF antenna, which may
also be called a RF transmitting antenna. The RF transmitting
antenna is operated, for example, by at least one RF amplifier. The
at least one RF amplifier transmits a transmission signal by a RF
transmitting voltage to the RF transmitting antenna. The
transmission signal has, for example, a sinusoidal shape having a
particular transmit frequency, which is limited by an envelope,
(e.g., a rectangular or Gaussian curve).
Scaling of the RF transmitting voltage for the RF pulses may be
based on a reference voltage obtained in advance by a scan on the
respective patient. During this scan, RF pulses are generated by
transmission signals whose transmit frequency is the center
frequency of the transmitting system. During recording of imaging
scan data that follows the scaling, the problem occurs of the image
quality may be inadequate in the case of excitation by RF pulses
that are played at a different transmit frequency to the center
frequency.
SUMMARY AND DESCRIPTION
The object of the present disclosure includes a method that
improves the image quality in the case of excitation by RF pulses
that are not played at the center frequency.
The scope of the present disclosure is defined solely by the
appended claims and is not affected to any degree by the statements
within this description. The present embodiments may obviate one or
more of the drawbacks or limitations in the related art.
A method for determining a transfer function of a transmitting
system of a magnetic resonance device is proposed. A transfer
function is determined using a transmission characteristic of a
transmitting system of the magnetic resonance device, wherein the
transfer function is frequency-dependent. A transmission signal may
be corrected using the transfer function. An excitation pulse may
be emitted by the transmitting system using the corrected
transmission signal.
In particular, a frequency-dependent deviation in the B.sub.1 field
may be corrected by the transfer function. In particular, it may be
achieved thereby that even in the case of RF pulses which are not
played at center frequency, the reference to a possible reference
voltage is correct, so the desired flip angle is excited in this
case as well. The image quality, (e.g., the image homogeneity), may
therefore be significantly improved. The determined transfer
function may be saved in a storage unit.
The transmitting system may include at least one RF transmitting
antenna, in particular, a body coil. Body coils, (e.g., whole-body
coils), may be permanently integrated in the magnetic resonance
device.
The transmission characteristic of the transmitting system may be
defined by structural properties, (e.g., the size and/or the form
and/or the construction), of the transmitting system. In other
words, the hardware configuration of the transmitting system as a
rule determines the transmission characteristic of the transmitting
system. The properties of the at least one RF transmitting antenna
may dominate the transmission characteristic of the transmitting
system, but other components in the transmission chain may also
have an effect thereon. The transmission characteristic of the
transmitting system, in particular, of the at least one RF
transmitting antenna, may be described by the bandwidth and/or the
frequency response of the transmitting system. The bandwidth of the
at least one RF transmitting antenna depends, in particular, on its
quality (also called the quality factor or Q-factor). As a rule,
the quality may be determined from the ratio of the resonance
frequency of the transmitting system to the bandwidth of the
transmitting system.
The transfer function H(f) may reflect a transmission of a RF
transmitting voltage U.sub.tra(f), which is applied to the at least
one RF transmitting antenna, in the generated B.sub.1 field
B.sub.1(f): H(f).varies.B.sub.1(f)/U.sub.tra(f). The frequency f
may also be described as the angular frequency .omega.=2.pi.f.
During determination of the transfer function, it is not usually a
matter of determining an absolute value for H(f). Instead, what is
crucial is the relative change in H(f) as a function of the
frequency f. Of particular interest is by what factor the generated
B.sub.1 field B.sub.1(f) changes with a change in f. This makes it
possible to calculate factors, for example, for two values f.sub.1
and f.sub.2, as B.sub.1(f.sub.1)/B.sub.1(f.sub.2).
A plurality of methods is conceivable for determining the transfer
function H(f). In particular, the transfer function is determined
using an absorbed power.
For example, the frequency dependency of the generated B.sub.1
field is determined, in particular empirically, by way of an input
reflection factor of the at least one RF transmitting antenna. If
the input reflection factor R(f) is known, H(f) may then be
approximated by H(f).varies.(1-|R(f)|.sup.2).sup.1/2. Here,
1-|R(f)|.sup.2 describes the absorbed power. The absorbed power,
and therewith the transfer function H(f), may therefore be
determined using the input reflection factor R(f).
The input reflection factor R(f) may be calculated from a ratio of
a returning, in particular reflected, voltage to a forward voltage.
The two voltages may be measured, for example, with the aid of
directional couplers.
A further embodiment of the method provides that the transfer
function is determined using a current in a RF transmitting
antenna. This embodiment is based on the knowledge that the
generated B.sub.1 field is proportional to the current I(f) in the
RF transmitting antenna, in other words B.sub.1(f).varies.I(f). By
measuring the current, more precisely: the current strength, the
transfer function H(f).varies.I(f)/U.sub.tra(f) may therefore also
be determined.
The transfer function, in particular, the current in the RF
transmitting antenna, may be determined using at least one pickup
probe. The at least one pickup probe may be arranged in the
vicinity of the RF transmitting antenna, for example, with an
embodiment of the RF transmitting antenna as a birdcage coil, on an
end ring of the birdcage coil.
The voltage U.sub.pu(f) induced in the at least one pickup probe is
proportional to the current I(f) in the RF transmitting antenna,
for example, in the end ring of a birdcage coil, and therewith also
proportional to the generated B.sub.1 field B.sub.1(f), in other
words U.sub.pu(f).varies.B.sub.1(f). The transfer function
H(f).varies.U.sub.pu(f)/U.sub.tra(f) may then in turn be determined
thereby.
A further embodiment provides that the transfer function, in
particular, the current in the RF transmitting antenna, is
determined using a forward and/or reflected voltage in a power
supply unit to the RF transmitting antenna. The power supply unit
may include at least one electrical line which connects the RF
transmitting antenna to the at least one RF amplifier. In
particular, a value U.sub.pu,coupling(f) may be obtained from a
measurement of a complex addition of the forward and reflected
voltage and this is in turn proportional to the current I(f) in the
RF transmitting antenna and therewith also to the generated B.sub.1
field B.sub.1(f), in other words
U.sub.pu,coupling(f).varies.B.sub.1(f). The transfer function is
then produced in particular from the following relationship:
H(f).varies.U.sub.pu,coupling(f)/U.sub.tra(f).
According to a further embodiment, the transfer function is
determined by a variation in an applied magnetic field. The applied
magnetic field may be a magnetic field for predicting the nuclear
spins deflected by the excitation pulses. As a rule, the applied
magnetic field B.sub.0 is proportional to the Larmor frequency
f.sub.L, in other words the frequency with which the nuclear spins
precess, or in other words f.sub.L=.gamma.B.sub.0, where .gamma. is
the gyromagnetic ratio. The frequency f.sub.L may also be varied by
way of the applied magnetic field B.sub.0 therefore. The
frequency-dependent transfer function H(f) may therefore be
determined by the variation in the applied magnetic field.
The applied magnetic field B.sub.0 may be generated by a main
magnet that includes, for example, at least one, in particular
superconducting, main magnet coil, and may be varied by the main
magnet itself and/or by at least one further magnet. The magnetic
field B.sub.0,m generated by the main magnet itself may be varied,
for example, by a change in the current flow through the at least
one main magnet coil.
The applied magnetic field B.sub.0 A is varied by the at least one
further magnet, for example, by generation of a further magnetic
field B.sub.0,off by the at least one further magnet, which further
magnetic field B.sub.0,off is overlaid with the magnetic field of
the main magnet, e.g., the main magnet field, B.sub.0,m, or in
other words B.sub.0=B.sub.0,m+B.sub.0,off. The at least one further
magnet may include at least one gradient coil. The further magnetic
field B.sub.0,off may therefore also be called a gradient offset.
The further magnetic field B.sub.0,off may be varied by the at
least one gradient coil, for example, by a change in a current flow
through the at least one gradient coil.
A radio frequency transmitting voltage U.sub.tra(f) may be adjusted
in the case of different applied magnetic fields such that an
amplitude of a magnetic resonance signal remains constant. The RF
transmitting voltage U.sub.tra(f) may be a voltage applied to the
RF transmitting antenna. The magnetic resonance signal may be
detected by a receiving antenna, which may also be identical to the
RF transmitting antenna and may be evaluated by an evaluation unit.
The transfer function may then be determined according to
H(f).varies.1/U.sub.tra(f) from the RF transmitting voltages
U.sub.tra(f=f.sub.L) adjusted for the different applied magnetic
fields B.sub.0.
The magnetic resonance signal, whose amplitude is evaluated for
determining the transfer function, may originate from just one
plane. This plane is advantageously located in the isocenter of the
magnetic resonance device, because the center point of the field of
view is also conventionally located here. In particular, the plane
is oriented perpendicularly to a longitudinal axis of the magnetic
resonance device. The longitudinal axis may be called the z-axis.
In this case, the position of the isocenter is defined at z=0, so
this condition also applies here for said plane. The longitudinal
axis may be a center line of a cylinder that describes the shape of
a patient-receiving region of the magnetic resonance device.
It is also conceivable for the magnetic resonance signals to
originate from different planes. For this purpose, for example, a
spatially varying gradient magnetic field, (also called just a
gradient field for short), B.sub.0,gra is applied, for example, by
the at least one gradient coil. A spatially varying, in particular,
linearly increasing or decreasing, applied magnetic field
B.sub.0(z)=B.sub.0,m(z)+B.sub.0,gra(z) is formed thereby, for
example, parallel to a z-axis. Consequently, different Larmor
frequencies f.sub.L(z)=.gamma.B.sub.0(z) also act parallel to the
axis z. However, in contrast to the method variant described above,
in which the magnetic resonance signals originate from just one
plane, it cannot be ruled out here that the spatial field
distribution of the applied magnetic field B.sub.0(z) falsifies the
result.
Furthermore, it is proposed that the transfer function is
determined using a particularly numeric simulation, for example,
with the aid of a computer.
It is therefore conceivable for the transfer function to be
determined using an absorbed power and/or using an input reflection
factor and/or using a current in a RF transmitting antenna and/or
using at least one pickup probe and/or using a forward and/or
reflected voltage in a power supply unit to the RF transmitting
antenna and/or by way of a variation in an applied magnetic field
and/or using a simulation.
It is proposed that the transfer function is determined just once,
for instance during the course of a tune-up of the magnetic
resonance device. This is primarily advantageous if the anticipated
effect on the frequency response due to the respective patient is
rather low.
It is also conceivable for the transfer function to be determined
several times, in particular, before each magnetic resonance
examination of a patient. Particularly high accuracy requirements
may be met thereby.
Furthermore, a method for the correction of a transmission signal
of a magnetic resonance device is proposed. A transfer function is
provided here which has been determined according to one of the
methods described above. The transmission signal is corrected using
the transfer function, moreover.
An excitation pulse may be emitted by the transmitting system using
the corrected transmission signal. The transmission signal is
advantageously corrected in such a way that a uniform flip angle
excitation occurs due to the emitted excitation pulse.
Correction of the transmission signal may be made, for example, by
way of a correction unit which may have one or more processors. The
transfer function may be provided from a storage unit, in which the
transfer function is stored.
One embodiment of the method provides that the transfer function is
standardized, and the excitation pulse is corrected using the
standardized transfer function.
For example, the inverse of the transfer function
G(f)=(H(f)).sup.-1 is formed and this is scaled so the maximum
thereof is equal to 1, in other words MAX(G(f))=1. This function
may then be used for a correction, (e.g., predistortion), of the
transmission signal U.sub.tra(f): U(f)=G(f)*U.sub.tra(f). Here,
U(f) is the corrected transmission signal and U.sub.tra(f) the
actually desired transmission signal in the frequency range.
A further embodiment of the method provides that the transfer
function is transformed, and the excitation pulse is corrected
using the transformed transfer function.
For example, the transfer function is transformed in a time domain.
For example, the inverse Fourier transform g(t) is formed from G(f)
for use in the time domain t. The transmission signal may be
corrected by a convolution U(t)=g(t)**U.sub.tra(t). As a rule, the
time characteristic of the desired transmission signal U.sub.tra(t)
is known, for which reason the convolution may be implemented with
the inverse transfer function.
One embodiment of the method provides that an applied magnetic
field is less than 1 Tesla (T). The applied magnetic field may be a
magnetic field around which the nuclear spins deflected by the
excitation pulses precess during the magnetic resonance scan. The
magnetic field may be composed of a main magnet field and a
gradient magnetic field that are overlaid. In particular, the
magnetic resonance device has a main magnet field of less than 1
T.
This aspect is based on the knowledge that the bandwidth of the
transmitting system is low with a low applied magnetic field.
Transmission signals, whose transmit frequency does not match the
center frequency of the transmitting system, are particularly
strongly falsified by the small bandwidth. The proposed correction
of the transmission signal using the transfer function is therefore
particularly advantageous in this case.
The transmitting system may have at least one superconducting,
(e.g., cooled), RF transmitting antenna. As a rule, RF transmitting
antennae of this kind have rather low bandwidths, for which reason
the correction of the transmission signal using the transfer
function is particularly advantageous here as well, in particular,
also in the case of applied magnetic fields of more than 1 T.
In one embodiment, a plurality of slices, (e.g., spaced apart
slices), is simultaneously excited by the transmitting system. The
slices may be spaced apart parallel to a magnetic field gradient,
for example, parallel to a longitudinal axis of the magnetic
resonance device.
With simultaneous excitation of a plurality of slices, RF pulses
having a different transmit frequency or a RF pulse having a
frequency spectrum, which includes portions spaced apart from the
center frequency, are generated. The different transmission
frequencies may be necessary to produce resonance conditions for an
effective excitation of the nuclear spins in the desired slices, in
which a different applied magnetic field is given.
The correction of the transmission signal using the transfer
function means that the nuclear spins having the same flip angle
are excited in the plurality of slices. Simultaneous excitation of
a plurality of slices may also be used in Simultaneous MultiSlice
(SMS) techniques.
A further embodiment of the method provides that the transmitting
system excites a field of view (FoV), which field of view has an
extent in a first direction. A gradient magnetic field is applied
in the first direction, for example, parallel to the longitudinal
axis of the magnetic resonance device. The product of the extent of
the field of view of the first direction and an amplitude of the
gradient magnetic field in the first direction is greater than 1
mT, greater than 2 mT, or greater than 5 mT.
With large fields of view, large slice shifts are possible which,
with high amplitudes of the gradient magnetic field, lead to large
frequency shifts, and therewith to large spacings from the center
frequency of the transmitting system. This may be considered by the
proposed correction of the transmission signal using the transfer
function.
Furthermore, a magnetic resonance device is proposed which is
designed to carry out a method for determining a transfer function
of a transmitting system of a magnetic resonance device and/or for
the correction of a transmission signal of a magnetic resonance
device.
The advantages of the magnetic resonance device match the
advantages of the method for determining a transfer function of a
transmitting system of a magnetic resonance device and/or of the
method for the correction of a transmission signal of a magnetic
resonance device, which have been stated above in detail. Features,
advantages, or alternative embodiments mentioned in this connection
may similarly also be transferred to the magnetic resonance device
and vice versa.
In other words, the concrete claims are also developed by the
features which are described or claimed in connection with a
method. The corresponding functional features of the method are
formed by corresponding concrete modules, in particular, by
hardware modules.
For the correction of the transmission signal, the magnetic
resonance device may include a correction unit which has, for
example, one or more processors. The magnetic resonance device may
include a storage unit in which the transfer function may be stored
and/or may be transmitted from there to the correction unit.
The magnetic resonance device may have units for determining the
transfer function, such as one or more pickup probes.
Furthermore, a computer program product is proposed that includes a
program and may be directly loaded into a storage device of a
programmable arithmetic unit of a correction unit and has program
modules, for example, libraries and auxiliary functions, in order
to carry out a method for determining a transfer function of a
transmitting system of a magnetic resonance device and/or a method
for the correction of a transmission signal of a magnetic resonance
device when the computer program product is run in the correction
unit. The computer program product may include software having a
source code, which still has to be compiled and linked or which
just has to be interpreted, or an executable software code which
just has to be loaded into the correction unit for execution. The
method for determining a transfer function of a transmitting system
of a magnetic resonance device and/or a method for the correction
of a transmission signal of a magnetic resonance device may be
carried out quickly, robustly, and in a way that may be repeated in
an identical manner by the computer program product. The computer
program product is configured in such a way that it may carry out
the method acts by the correction unit. The correction unit has the
requirements in each case, (e.g., an appropriate working memory
and/or an appropriate logic unit), so the respective method acts
may be carried out efficiently.
BRIEF DESCRIPTION OF THE DRAWINGS
Further advantages, features, and details of the disclosure emerge
from the exemplary embodiments described below and with reference
to the drawings. Mutually corresponding parts are provided with the
same reference characters in all figures, in which:
FIG. 1 depicts a magnetic resonance device in a schematic
diagram.
FIG. 2 depicts a block diagram of a method for determining a
transfer function of a transmitting system of a magnetic resonance
device and/or a method for the correction of a transmission signal
of a magnetic resonance device.
FIG. 3 depicts an exemplary construction of a RF transmission
chain.
FIG. 4 depicts a graph with exemplary frequency-dependent impedance
curves of a RF transmitting antenna in the case of different
applied magnetic fields.
DETAILED DESCRIPTION
FIG. 1 schematically depicts a magnetic resonance device 10. The
magnetic resonance device 10 includes a magnetic unit 11, which may
include a superconducting, main magnet 12 for generating a strong
main magnet field, which is, in particular, constant over time. The
magnetic resonance device 10 includes a patient-receiving region 14
for receiving a patient 15. In the present exemplary embodiment,
the patient-receiving region 14 is cylindrical and cylindrically
surrounded in a circumferential direction by the magnetic unit 11.
A different design of the patient-receiving region 14 is also
conceivable.
The patient 15 may be pushed by a patient-positioning device 16 of
the magnetic resonance device 10 into the patient-receiving region
14. The patient-positioning device 16 has for this purpose an
examination table 17 designed so it may be moved inside the
patient-receiving region 14.
The magnetic unit 11 has at least one, (e.g., three), gradient
coils 18 for generating a gradient magnetic field, which is used
for spatial encoding during imaging. The gradient magnetic field
and the main magnet field are overlaid to form an applied magnetic
field B.sub.0. The at least one gradient coil 18 is controlled by a
gradient control unit 19 of the magnetic resonance device 10.
The magnetic unit 11 includes a RF transmitting antenna 2, which in
the present exemplary embodiment is designed as a body coil
permanently integrated in the magnetic resonance device 10. The RF
transmitting antenna 20 is designed for excitation of nuclear spins
which are established in the applied magnetic field B.sub.0.
The RF transmitting antenna 20 is controlled by a RF antenna
control unit 21 of the magnetic resonance device 10 in that
transmission signals are transmitted by the RF antenna control unit
21 to the RF transmitting antenna 20. By the transmission signals,
RF pulses are generated by the RF transmitting antenna 20, and the
RF pulses are irradiated into an examination space formed by a
patient-receiving region 14 of the magnetic resonance device 10.
The irradiated RF pulses cause a magnetic alternating field, which
may also be called a B.sub.1 field, in the patient-receiving region
14.
The RF transmitting antenna 20 and/or the RF antenna control unit
21 are therefore parts of a transmitting system of the magnetic
resonance device 10, which have a transmission characteristic
dependent on the frequency of the RF pulses. The RF transmitting
antenna 20 may also be designed for receiving magnetic resonance
signals.
The magnetic resonance device 10 has a system control unit 22 for
controlling the main magnet 12, the gradient control unit 19, and
the RF antenna control unit 21. The system control unit 22
centrally controls the magnetic resonance device 10, such as
carrying out a predetermined imaging pulse sequence. The system
control unit 22 includes a correction unit 26 and a storage unit
27. With the aid of the correction unit 26, a frequency-dependent
transfer function may be determined using a transmission
characteristic of a transmitting system which may optionally be
stored in the storage unit 27, and/or a transmission signal may be
corrected using the transfer function. A corresponding computer
program product may be run in the correction unit for this
purpose.
In addition, the system control unit 22 includes an evaluation unit
(not shown) for an evaluation of medical image data which is
acquired during the magnetic resonance examination. Furthermore,
the magnetic resonance device 10 includes a user interface 23,
which is connected to the system control unit 22. Control
information, (e.g., imaging parameters), and reconstructed magnetic
resonance images may be displayed on a display unit 24, (e.g., on
at least one monitor), of the user interface 23 for medical staff.
The user interface 23 also has an input unit 25 by which the
medical staff may input information and/or parameters during a
scanning process.
FIG. 2 depicts a block diagram of a method for determining a
transfer function of a transmitting system of a magnetic resonance
device and/or a method for the correction of a transmission signal
of a magnetic resonance device. In act 100, a frequency-dependent
transfer function is determined using a transmission characteristic
of the transmitting system of the magnetic resonance device 10.
In optional act 110, the determined transfer function is provided
and used for a correction of a transmission signal. In a further
optional act 120, an excitation pulse is emitted by the
transmitting system using the corrected transmission signal.
The transfer function H(f) reflects a transmission of a RF
transmitting voltage U.sub.tra(f) into a generated B.sub.1 field
B.sub.1(f): H(f).varies.B.sub.1(f)/U.sub.tra(f).
A plurality of procedures is conceivable for determining this
transfer function in act 100, in particular for frequencies close
to the center frequency of the transmitting system. For example,
the transfer function may theoretically be determined using a
simulation.
The transfer function H(f) may also be determined empirically. For
example, the frequency dependency of the generated B.sub.1 field is
determined by the root of an absorbed power which may be given with
the aid of an input reflection factor R(f) of the RF transmitting
antenna 20 according to H(f).varies.(1-|R(f)|.sup.2).sup.1/2.
The transfer function H(f) may also be determined with the aid of
an electrical current in the RF transmitting antenna 20. The fact
that in the frequency range of interest, the generated B.sub.1
field is proportional to the current in the RF transmitting antenna
20 is used here. H(f) may also be determined by way of measurement
of the current.
Two variants of a current measurement are illustrated with
reference to FIG. 3. The current may be measured on the one hand by
way of a pickup probe 30, which is placed in the vicinity of the RF
transmitting antenna 20, formed here as a birdcage coil. The pickup
probe 30 is arranged here in the vicinity of an end ring 35 of the
birdcage coil 20. The voltage U.sub.pu induced in the pickup probe
30 is proportional to the current in the end ring 35 and therewith
also proportional to the generated B.sub.1 field:
H(f).varies.U.sub.pu(f)/U.sub.tra(f).
Alternatively, or additionally, for measurement by pickup probes
30, a value U.sub.pu,coupling(f), which is also again proportional
to the current in the RF transmitting antenna 20 and therefore the
generated B.sub.1 field: U.sub.pu,coupling(f).varies.B.sub.1(f),
may also be obtained from a measurement of a complex addition of a
forward voltage U.sub.F,1, U.sub.F,2 and reflected voltage
U.sub.R,1, U.sub.R,2 in electrical lines L.sub.1 and L.sub.2, which
connect the RF transmitting antenna 20 to a RF power amplifier
32.
FIG. 3 depicts further possible components of a RF transmission
chain, such as, for instance, a 90.degree. hybrid coupler 31, a
terminating resistor 34, and further elements 33 of the RF
transmission chain.
A further method is based on a measurement of the B.sub.1 field
using a recording of magnetic resonance signals. An operating
frequency of the magnetic resonance device 10 is changed in the
process in order to determine the frequency dependency of the
transfer function. The operating frequency of the magnetic
resonance device 10 may be changed by a variation in the applied
magnetic field B.sub.0. The applied magnetic field B.sub.0 may be
changed by a change in the main magnet field and/or the gradient
magnetic field. To determine the transfer function H(f), the RF
transmitting voltages U.sub.tra(f) are determined for different
strengths of the applied magnetic field B.sub.0, at which voltages
the amplitudes of the magnetic resonance signals remain constant.
The following is obtained thereby: H(f).varies.1/U.sub.tra(f).
Magnetic resonance signals may be evaluated from just one plane,
such as, in particular, the plane z=0 shown in FIG. 1. The z-axis
is a longitudinal axis of the magnetic resonance device, which is
located centrally here in the cylindrical patient-receiving region
14 of the magnetic resonance device 10.
Furthermore, it is conceivable to carry out the measurement of the
B.sub.1 field in different planes, with the different planes
corresponding to different frequencies. The spatial field
distribution is also included in the result here, however.
The transfer function H(f) may be used either in the frequency
range or particularly advantageously in the time domain. In both
cases, the inverse of the transfer function G(f)=(H(f)).sup.-1 may
be formed first and scaled such that MAX(G(f))=1. This function is
then used for correction of the transmission signals, whereby a
smooth frequency response is achieved: U(f)=G(f)*U.sub.tra(f). U(f)
is the corrected transmission signal and U.sub.tra(f) the actually
desired transmission signal in the frequency range.
For use in the time domain, the inverse Fourier transform g(t) of
G(f) may be formed first. The transmission signal may then be
corrected by a convolution U(t)=g(t)*U.sub.tra(t). As a rule, the
time characteristic of the desired transmission signal U.sub.tra(t)
is known, for which reason the convolution may be implemented with
the inverse transfer function.
If particularly high accuracy requirements are placed on the
correction, the transfer function is advantageously determined for
each patient. With less high accuracy requirements and/or where the
patient has less of an effect on the frequency response, detection
of the transfer function with an apparatus of the magnetic
resonance device may be sufficient.
The correction in act 110 may be used in the case of applied
magnetic fields B.sub.0 of less than 1 T. The method may be
employed under this condition because the bandwidth of the RF
transmitting antenna 20 becomes narrower while retaining the same
quality. This connection will be illustrated in more detail with
reference to FIG. 4. This shows by way of example an input
impedance of a RF transmitting antenna 20 having a quality of 200.
A deviation from the center frequency in kHz is plotted on the
horizontal axis, while the input impedance standardized to 1 is
plotted on the vertical axis. The graph includes two curves that
show the characteristics of an applied magnetic field of 0.5 T and
1.5 T. The curve for 0.5 T is significantly more narrowband. The -3
dB bandwidth is only 110 kHz, in other words with a frequency
offset of 55 kHz the generated B.sub.1 field is already reduced by
3 dB compared to the maximum.
With low field strengths, the fact that the patient 15 causes only
slight loading of the RF transmitting antenna 20 also comes into
effect, whereby the bandwidth of the RF transmitting antenna
remains the same. This is different in the case of high field
systems in which the bandwidth is significantly increased by the
patient load.
Even in systems with superconducting RF transmitting antennae, the
correction of the transmission signal is beneficial because
superconducting RF transmitting antennae may be quite
narrow-band.
In addition, the correction of the transmission signal may be used
particularly advantageously with simultaneous excitation of a
plurality of slices. For example, FIG. 1 shows two slices S1 and
S2. These slices are located in different planes here perpendicular
to the longitudinal axis z of the magnetic resonance device 10, so
when a gradient magnetic field is applied parallel to this
longitudinal axis z, different applied magnetic fields act in these
planes. To excite the nuclear spins in these slices, it is
therefore necessary for the RF pulses generated using the
transmission signal to include different frequencies and/or for the
frequency spectrum of a RF pulse to have portions with a different
frequency. Without correction the excitation in the different
slices S1 and S2 would lead to a different flip angle excitation of
the nuclear spins.
The correction of the transmission signal has an advantageous
effect even with a large field of view. FIG. 1 also illustrates an
extent of a field of view FoV.sub.z parallel to the longitudinal
axis z of the magnetic resonance device 10. If a gradient magnetic
field is applied in this direction, the otherwise adverse effects
of a frequency offset caused thereby may be compensated in this
direction by a correction of the transmission signal. For example,
a frequency offset of 170 kHz results with a slice shift of 200 mm
at an amplitude of the gradient magnetic field of 20 mT/m.
To conclude, reference is made to the fact that the methods
described in detail above and the illustrated magnetic resonance
device are merely exemplary embodiments which may be modified in a
wide variety of ways by a person skilled in the art without
departing from the scope of the disclosure. Furthermore, use of the
indefinite article "a" or "an" does not preclude the relevant
features from also being present multiple times. Similarly, the
term "unit" does not preclude the relevant components from
including a plurality of cooperating sub-components which may
optionally also be spatially distributed.
It is to be understood that the elements and features recited in
the appended claims may be combined in different ways to produce
new claims that likewise fall within the scope of the present
disclosure. Thus, whereas the dependent claims appended below
depend from only a single independent or dependent claim, it is to
be understood that these dependent claims may, alternatively, be
made to depend in the alternative from any preceding or following
claim, whether independent or dependent, and that such new
combinations are to be understood as forming a part of the present
specification.
While the present disclosure has been described above by reference
to various embodiments, it may be understood that many changes and
modifications may be made to the described embodiments. It is
therefore intended that the foregoing description be regarded as
illustrative rather than limiting, and that it be understood that
all equivalents and/or combinations of embodiments are intended to
be included in this description.
* * * * *